When I was first learning about dermatology I remember being told about the skin as an “acid mantle” which was key to keeping pathological microbes from the skin. Although discussed in textbooks, little research has appeared examining the acidic function of the epidermis. As a podiatrist, skin infection is a common problem and so I thought in this blog I would look at the topic.
The epidermis consists of distinct layers of keratinocytes which function cohesively to maintain the skin as an effective barrier against a range of threats including infection. A key part of this biological barrier is the acidic nature of the outermost layer the stratum corneum. The term “acid mantle” was first used by Schade in 1928 [1] building on the work of others who had recognised an acidic skin surface and in his paper was proposed it existed as an antimicrobial barrier mechanism. Although correct, it doesn’t cover the full extent of its true function according to our current understanding.
What is the normal pH value of the skin?
The pH of skin is around 5.5 (range 4.1- 5.8) as measured on the surface of the stratum corneum. The importance of preserving this pH level is now appreciated in normal skin physiology and the development of topical agents – which goes further than just an antimicrobial barrier.
Descending through the lower layers of the stratum corneum (SC) the pH then rises before dropping again in the stratum granulosum gradually equalising to the body internal pH of around 7.0, deeper in the epidermis.
The skins natural pH varies by anatomical location [2] although all areas of the skin have not been fully mapped, measurements of the leg and foot showed pH varied between 4.5 – 5.5 (calf, heel and arch) [3].
What is the function of this lower pH?
Skin physiology is well adapted to function at this lower than body pH. An acidic environment is important for enzyme function - vital to allow the manufacture and assembly of lipid layers in the epidermis which give the waterproofing necessary to prevent dehydration. To an extent this waterproofing is facilitated by the skins production of natural moisturising factors (NMFs) by the degradation of Filaggrin into several components including pyrrolidone carboxylic Acid (PCA), amino-acids and urocanic acid (UA).
Raising of the pH would affect their assembly lead to an imperfect barrier and increase in water loss and dryness. For example, ceramides, the moist abundant skin lipid is manufactured in an acidic environment. Raising of the pH can lead to their disruption and depletion from the epidermis.
Acidity also ensures an environment hostile to common pathogens which typically function best at a neutral pH, limiting their activity on the skin, whilst competing with the natural flora and fauna present on the surface [4].
Finally, a low pH suppresses metalloprotease enzymes within the skin. Raising of the pH increases enzymatic activity leading to skin desquamation and ultimately thinning of the skin, again decreasing its barrier properties and increasing potential complications such as infection.
Changes in pH with age
As we age, skin function decreases. It has been shown older adults have an increase in skin pH (5.5-6.0) [5]. An temporary increase in pH probably has little effect but long term it can major affects degrading lipid assembly and function as well as increasing skin permeability [6] leading to potential irritation and allergy.
Skin disease and pH
Whilst changes in skin pH do not lead to specific skin disorders, a more alkaline skin environment can lead to dysfunction and increased skin permeability and water loss (dry skin). Atopic dermatitis is a good example of a skin disease with pH disruption. This can occur through reduction of filaggrin production and subsequent reduction in UA and PCA, natural moisturising factors produced in the skin. Around 50% of AD sufferers have Filaggrin (FLG) gene mutations reducing FLG production, resulting in a higher skin pH. This can be exacerbated using topical alkaline skin agents such as soaps and sodium lauryl sulphate containing products.
Ichthyosis vulgaris, an inherited skin condition, which I covered in an earlier blog, represents a mutation in the FLG gene resulting in a reduced production in acidifying components in keratinocytes, again leading to increased skin pH.
Diabetes and skin pH
The effects of Diabetes Mellitus on skin pH has been investigated but is not fully understood. The acidity of the skin may depend on the type of diabetes (type 1 or type 2) [7]. In type 2 skin, pH has been found to be raised versus non-diabetic controls in some studies. Although only limited specific anatomical sites have been assessed such as intertriginous areas [8], it may help to explain why patients with type 2 diabetes are more susceptible to bacterial skin infections. Although, fungal skin infection is probably more common in diabetes [9-11], no research has investigated if altered skin pH plays a part in this observed phenomenon.
Implications for practice
From our knowledge to date, maintaining an acidic pH is important for a number of reasons, but can easily be summarised as maintaining skin integrity reducing skin dryness, infection and inflammation. A higher pH due to ageing or diabetes for example can lead to the skin be more readily affected by skin infection or irritation and inflammation.
Recent literature has focussed on the concept that disruption of the skin barrier promotes inflammation and ageing (often coupled and termed “inflammageing” of the skin) [12]. Reduced barrier function coupled with a reduction in immune functions can lead to chronic low-grade inflammation in the epidermis driven by cytokines such as IL-6, IL-8 and TNF-α. The effects of this inflammation can be reduced wound healing and resolution following inflammation.
Empirically, management for most patients is straightforward. The use of emollients can be helpful to restore pH. For example, one study has demonstrated the rapid effect of a low pH moisturiser in normalising skin pH in older adults within hours, improving barrier function [5].
Lowering skin pH is likely to have multiple effects but particularly allowing assembly of natural skin lipids to re-establish a lipid barrier reducing water loss, dryness and itch in addition to reducing inflammation driven by a higher pH.
Ultimately, this highlights another dimension for the regular use of emollients – not just to treat dry skin, but if the science is correct, to reduce the pH whilst also reducing the likelihood of further skin problems driven by chronic, low grade inflammation.
References
1. Schade, M.A., Sauremantel der haut nach gaskettenmessngen. J Mol Med, 1928. 7: p. 12-14.
2. van Hemmen, J.J. and C.L. Packham, Essentials of occupational skin management. A practical guide to the creation and maintenance of an effective skin management system. Occup Hyg, 2000(44): p. 77.
3. Kleesz, P., R. Darlenski, and J.W. Fluhr, Full-Body Skin Mapping for Six Biophysical Parameters: Baseline Values at 16 Anatomical Sites in 125 Human Subjects. Skin Pharmacology and Physiology, 2011. 25(1): p. 25-33.
4. Korting, H.C., et al., Influence of the pH-value on the growth of Staphylococcus epidermidis, Staphylococcus aureus and Propionibacterium acnes in continuous culture. Zentralbl Hyg Umweltmed, 1992. 193(1): p. 78-90.
5. Blaak, J., R. Wohlfart, and N. Schürer, Treatment of Aged Skin with a pH 4 Skin Care Product Normalizes Increased Skin Surface pH and Improves Barrier Function: Results of a Pilot Study. Journal of Cosmetics, Dermatological Sciences and Applications, 2011. 1(3).
6. Hachem, J.-P., et al., Sustained Serine Proteases Activity by Prolonged Increase in pH Leads to Degradation of Lipid Processing Enzymes and Profound Alterations of Barrier Function and Stratum Corneum Integrity. Journal of Investigative Dermatology, 2005. 125(3): p. 510-520.
7. Poonja, P., Skin barrier changes in T1DM and T2DM. International Journal of Diabetes and Clinical Research, 2022. 9(2).
8. Yosipovitch, G., et al., Skin surface pH in intertriginous areas in NIDDM patients. Possible correlation to candidal intertrigo. Diabetes Care, 1993. 16(4): p. 560-3.
9. Eckhard, M., et al., Fungal foot infections in patients with diabetes mellitus - results of two independent investigations. Mycoses, 2007. 50(s2): p. 14-19.
10. Navarro-Pérez, D., et al. Prevalence and Risk Factors Predicting Onychomycosis in Patients with and Without Diabetes Mellitus in Spain: A Cross-Sectional Study. Journal of Fungi, 2024. 10, DOI: 10.3390/jof10110790.
11. Rich, P., Onychomycosis and tinea pedis in patients with diabetes. Journal of the American Academy of Dermatology, 2000. 43(Supp 1): p. S130 -134.
12. Agrawal, R., A. Hu, and W.B. Bollag The Skin and Inflamm-Aging. Biology, 2023. 12, DOI: 10.3390/biology12111396.